The movement of myoglobin out of myocytes and intracellular diffusion of IgG will be studied in myocardial ischemia (MI). Preliminary data in dogs indicate that both types of molecules begin to diffuse soon after the onset of MI and that they are markers of irreversible injury to muscle cells. In these studies the movement of myoglobin and IgG cell membranes will be correlated with known criteria for establishing irreversible injury of cells. From this we hope to prove that one or both events can be used by themselves to identify irreversibly injured cells well before their injury is evident by means now applicable to human autopsy material. In addition, the serum level of myoglobin after myocardial ischemia will be demonstrated by radioimmunoassay to determine how rapidly it rises and how long the peak is maintained. Both of these parameters are of course important in determining how useful myoglobinemia will be in following myocardial ischemia. The circumflex branch of the left coronary of dogs will be occluded by a closed chest technique for 0.5 to 6 hours. Coronary flow will be determined before, during, and at the end of each experiment using radio-labeled microspheres. Myoglobin and IgG will be demonstrated by indirect immunofluorescence in fixed paraffin-embedded and also EM prepared material along with glycogen.